There was once a truly miserable land where everyone was depressed. There was no reason for this to be so: the people were normal, and they led normal lives, but they were very sad. One day, a baby boy was born. He was a really good child; always thinking of others, always clean and tidy, always quick to do as he was told… he was equally good in all such matters. As a result, the boy managed to cheer up everyone around him. People saw him and wanted to be like him, so they behaved cheerfully, and, in turn, cheered up others. The boy travelled the whole land, bringing smiles to people's faces, and happiness to their hearts. He ended up transforming the entire country from one of sadness and depression to one which could even be called happyland. 
A pervasive negative atmosphere can be dispelled by the presence of one infectiously cheerful individual. 